Renamed interaction_variable_get_var to interaction_get_variable.

Renamed interaction_variable_get_var to interaction_get_variable.

Renamed interaction_variable_get_member to interaction_get_member.

Split update_hash_entry into update_hash_entry and
update_hash_entry_intr for interactions.

inner_intr_loop: New function.

covariance_accumulate_pairwise: Loop separately over variables, then interactions.

interaction_variable_create: Make interactions type alpha when
appropriate.

interaction_value_create: Use value_resize to avoid copying more data than
necessary into new interaction_value.